```
public int minSubArrayLen(int s, int[] a) {
if (a == null || a.length == 0)
return 0;
// i is slow pointer, j is fast pointer
int i = 0, j = 0, sum = 0, min = Integer.MAX_VALUE;
while (j < a.length) {
sum += a[j];
// keep adding a[j] till sum >= s
while (sum >= s) {
min = Math.min(min, j - i + 1);
// keep subtracting a[i] till sum < s
sum -= a[i++];
}
j++;
}
return min == Integer.MAX_VALUE ? 0 : min;
}
```